This disclosure relates to quadrature signal processing of local oscillator and Doppler frequency-shifted signals in a lidar or other coherent optical systems.
A primary obstacle of fiber lidar is assumed to be the birefringent depolarization of the local oscillator (LO) signal from the transmitted carrier after splitting from the lidar output path. The effect can destroy the heterodyne efficiency at the detector and hence lidar operation unless polarization preserving fiber is utilized in the system past the split point in homodyne systems. This effect is assumed worse in heterodyne systems utilizing different LO and transmitter sources. The only form of the optical fiber lidar xe2x80x9cimmunexe2x80x9d from this effect utilizes a local oscillator signal taken from the Fresnel reflection at the end of the transmit fiber immediately preceeding the output telescope. However, this latter mode of operation is not required as conventionally assumed. Laboratory tests have shown that phonon modulation of the birefringence in the local oscillator path gives rise to AM modulation of the detected signals within the dynamic range required of the lidar to perform its basic task. This provides a statistically detectable signal.
Furthermore, in conventional lidar systems, a frequency offset between a local oscillator signal and a transmitted beam has been traditionally required. This has traditionally been achieved in homodyne operation via a frequency shifting device such as an expensive acousto-optic (A/O) modulator, or in heterodyne operation by maintaining a fixed offset between the frequencies of the two coherent sources. It is desirable to perform such heterodyning or homodyning without the use of such acousto-optic modulators.
The disclosed invention can be used in free-space lidar systems, fiber lidar systems, and other systems based upon coherent mixing to eliminate the costly A/O cell used for offset homodyne operation or the difficult to stabilize offset heterodyne source. These elements are replaced with inexpensive detectors and couplers with savings of several thousands of dollars. The use of the disclosed invention allows the effective use of non-polarized or polarization preserving fibers, depending on the coherent system design requirements. The disclosed invention can be utilized effectively in the presence of birefringent de-polarization.
Signal to noise ratio for the disclosed technique is within 3 dB of that engendered by the use of the typical A/O cell, but alignment and temperature sensitivities are considerably reduced. Further, the bandwidth requirements necessary in the processing electronics are cut in half relative to the A/O modulator or offset heterodyne systems. Lastly, the electronic support components required for the other system forms are eliminated with considerable savings in volume and electronic power. The use of multiple coherent wavelengths can be achieved with this disclosed invention
The disclosed technique enables considerably more compact systems to be fabricated and cost effectively extends the applicability of the typical fiber lidar into a wider range of applications that require fall signed Doppler spectrum (vector velocity). Typical applications that will see substantial benefit include vibration sensing, turbulence sensing and velocity lidars (e.g. police radar applications, relative motion sensing applications, optical air data systems, etc.) of any type (e.g. linear velocity, tangential velocity, spin sensing, etc.)